Want to add particle systems to areas of object intersection
During the creation of Sintel the "Grease Scatter Objects" addon was created and is included in every blender release since, you just need to enable it. You can watch a video of it in use with some more explanation and download an example blend if you wish.


create a variety of objects you want to scatter
add them all to the same group &#9096; CtrlG
draw some grease pencil strokes on your ground. Ensure your stroke is attached to the object surface not the scene.





select your ground mesh
select Add->Mesh->Grease Pencil Scatter and fill in your group name.